Traditional methods of applying a tone scale function to a digital image modify the apparent sharpness of the image because the tone scale modifies the amplitudes of high frequency detail information. This phenomenon occurs when the tone scale function is applied to each of the color channels independently, or when the tone scale function is applied to the luminance channel of a luminance-chrominance transformed digital image.
For systems in which the spatial filtering step must precede the application of a tone scale function, the resultant image detail will also be adversely modified with respect to the original image. The tonal characteristics of an output device, such as a display monitor and a digital printer, can have the same affect on image detail as an applied tone scale function. For these applications, any spatial filtering operations designed to compensate for the device characteristics must be applied before the image is passed to the device.
The prior art contains several examples which address the problem of detail distortion by tone scale application. Many of these examples utilize frequency decomposition methods in the application of the tone scale function.
For example, to apply a tone scale function to a digital image without distorting the detail information, in U.S. Pat. No. 5,012,333, Lee et al. proposed separating the image into a high frequency and a low frequency image by using FIR filters. The tone scale function is then applied to only the low frequency image, and the high frequency image is added back to the tone scaled low frequency image. Also, in U.S. Pat. No. 5,454,044, Nakajima suggests modifying the image contrast by the formula EQU Sproc=Sorg+f(Sus).
The low frequency image (Sus) is passed through function f() which is a monotonically decreasing function. This signal is added to the original (Sorg) to create the processed image Sproc. Both of these methods apply a tone scale function while attempting to preserve the image detail.
In addition, methods exist in the prior art to improve the detail of a digital image. These methods often utilize unsharp masking, which is well known in the art. Examples exist in the prior art that modify parameters of the unsharp masking adaptively while processing the digital image. For example, in U.S. Pat. No. 5,081,692, Kwon et al describes a method of modifying the gain of an unsharp mask based upon a local center weighted variance. However, none of the prior art methods of sharpening a digital image compensate specifically for the degradation to the image detail that is induced by an applied tone scale function.
In addition, it is well know in the art that various sharpening algorithms may be used to compensate for the loss of spatial detail due to blurring when printing or displaying an image on an output device. This method includes tailoring the sharpening to the spatial characteristics of the output device. However, it would also be beneficial to tailor the level of sharpening to the tonal response of the device as well.
All methods described in the prior art designed to modify the spatial detail in relation to a tone scale function are methods of tone scale function application. As such, the detail of the original image input to the tone scale function application algorithm has not yet been undesirably altered. The goal of these methods is the application of a tone scale function without damaging image detail. For applications in which a tone scale function has already been applied to an image the image detail has likewise already been modified. Thus the methods described in the prior art are not applicable to overcoming this shortcoming.
Consequently, a need exists for overcoming the above-described drawbacks. More specifically, a need exists for a method of restoring image spatial detail in an image processing step occurring after the application of a tone scale function. Also, a need exists for a method of modifying the image spatial detail in preparation for the anticipated application of a tone scale function.